A major source of methane is natural gas which typically contains about 85% methane and about 10% ethane with the balance being made up of propane, the butanes, the pentanes and nitrogen. The term "higher order hydrocarbon" refers to a hydrocarbon having at least two carbon atoms.
Primary sources for natural gas are the porous reservoirs generally associated with crude oil reserves. From these sources come most of the natural gas used for heating purposes. Quantities of natural gas are also known to be present in coal deposits and are byproducts of crude oil refinery processes and bacterial decomposition of organic matter. Natural gas obtained from these sources is generally utilized as a fuel at the site.
Prior to commercial use, natural gas must be processed to remove water vapor, condensible hydrocarbons and inert or poisonous constituents. Condensible hydrocarbons are generally removed by cooling natural gas to a low temperature and then washing the natural gas with a cold hydrocarbon liquid to absorb the condensible hydrocarbons. The condensible hydrocarbons are typically ethane and heavier hydrocarbons. This gas processing can occur at the wellhead or at a central processing station. Processed natural gas typically comprises a major amount of methane, and minor amounts of ethane, propane, the butanes, the pentanes carbon dioxide and nitrogen. Generally, processed natural gas comprises from about 50% to more than about 95% by volume of methane. Natural gas is used principally as a source of heat in residential, commercial and industrial service.
Methane has a number of commercial uses in the chemical processing industry. The largest use of methane, other than as a primary fuel, is in the production of ammonia and methanol. Ammonia is a basic ingredient of fertilizers and is also a common feedstock in the production of petrochemicals such as acrylonitrile and nylon-6. Methanol is a precursor material for products such as formaldehyde, acetic acid and polyesters. Methane has also been used as a feedstock for the production of acetylene by electric-arc or partial-oxidation processes. Another commercial use for methane is in the production of halogenated products such as methyl chloride, methylene chloride, chloroform and carbon tetrachloride. Methane also reacts with ammonia to produce hydrogen cyanide.
Most processed natural gas is distributed through extensive pipeline distribution networks. As natural gas reserves in close proximity to gas usage decrease, new sources that are more distant require additional transportation. Many of these distant sources are not, however, amendable to transport by pipeline. For example, sources that are located in areas requiring economically unfeasible pipeline networks or in areas requiring transport across large bodies of water are not amendable to transport by pipeline. This problem has been addressed in several ways. One such solution has been to build a production facility at the site of the natural gas deposit to manufacture one specific product. This approach is limited as the natural gas can be used only for one product, preempting other feasible uses. Another approach has been to liquefy the natural gas and transport the liquid natural gas in specially designed tanker ships. Natural gas can be reduced to 1/600th of the volume occupied in the gaseous state by such cryogenic processing, and with proper procedures, safely stored or transported. These processes, which involve liquefying natural gas to a temperature of about -162.degree. C., transporting the gas, and revaporizing it are complex and energy intensive.
Still another approach has been the conversion of natural gas to higher order hydrocarbons that can be easily handled and transported, preferably substantially liquid hydrocarbons. The conversion of natural gas to higher order hydrocarbons, especially ethane and ethylene, would retain the material's versatility for use as precursor materials in chemical processing. Known dehydrogenation and polymerization processes are available for the further conversion of ethane and ethylene to liquid hydrocarbons. In these ways, easily transportable commodities may be derived directly from natural gas at the wellhead. A drawback in implementing such processes has been in obtaining a sufficient conversion rate of natural gas to higher order hydrocarbons.
The conversion of methane to higher order hydrocarbons at high temperatures, in excess of about 1200.degree. C. is known. These processes are, however, energy intensive and have not been developed to the point where high yields are obtained even with the use of catalysts. Catalysts that are useful in these processes (e.g., chlorine) are corrosive under such operating conditions.
The catalytic oxidative coupling of methane at atmospheric pressure and temperatures of from about 500.degree. to 1,000.degree. C. has been investigated by G. E. Keller and M. M. Bhasin. These researchers reported the synthesis of ethylene via oxidative coupling of methane over a wide variety of metal oxides supported on an alpha alumina structure in Journal of Catalysts 73, 9-19 (1982). This article discloses the use of single component oxide catalysts that exhibited methane conversion to higher order hydrocarbons at rates no greater than four percent. The process by which Keller and Bhasin oxidized methane was cyclic, varying the feed composition between methane, nitrogen and air (oxygen) to obtain higher selectivities.
West German Patent No. DE 32370792 discloses the use of single supported component oxide catalysts. The process taught by this reference utilizes low oxygen partial pressure to give a high selectivity for the formation of ethane and ethylene. The conversion of methane to ethane and ethylene is, however, only on the order of from about four to about seven percent.
Methods for converting methane to higher order hydrocarbons at temperatures in the range of about 500.degree. to about 1,000.degree. C. are disclosed in U.S. Pat. Nos. 4,443,644; 4,443,645; 4,443,646; 4,443,647; 4,443,648; and 4,443,649. The processes taught by these references provide relatively high selectivities to higher order hydrocarbons but at relatively low conversion rates, on the order of less than about four percent overall conversion. In addition to synthesizing hydrocarbons, the processes disclosed in these references also produced a reduced metal oxide which must be frequently regenerated by contact with oxygen. The preferred processes taught by these references entail physically separate zones for a methane contacting step and for an oxygen contacting step, with the reaction promoter recirculating between the two zones.
U.S. Pat. Nos. 4,495,374 and 4,499,322 disclose processes for converting methane to higher order hydrocarbons using an oxidative synthesizing agent containing an alkali metal or compound thereof as a promoter. Both patents indicate that stability of the promoted synthesizing agent is enhanced by the presence of phosphorous.
Phosphate-containing catalysts have been disclosed for use in dehydrogenation process. For example, U.S. Pat. No. 4,255,283 discloses a process for the oxydehydrogenation of alkyl-substituted aromatic compounds to the corresponding alkenyl-substituted aromatics using a catalyst represented by the formula EQU A.sub.a M.sub.b M.sup.1.sub.c M.sup.11.sub.d B.sub.e P.sub.y O.sub.x
wherein A is an alkali metal and/or thallium; M is one or more of the elements of nickel, cobalt, copper, manganese, magnesium, zinc, calcium, niobium tantalum, strontium or barium; M.sup.1 is one or more of the elements of iron, chromium, uranium, thorium, vanadium, titanium, lanthanum or the other rare earths; M.sup.11 is one or more of the elements of tin, boron, lead, germanium, aluminum, tungsten or molybdenum; B is bismuth, tellurium, arsenic, antimony, cadmium, or combinations thereof; P is phosphorus; and wherein a through y have the following values: a=0 to 20; b=0 to 20; c=0 to 20; d=0 to 4; e=0.1 to 20; y=8 to 16; x=the number of oxygens required to satisfy the valence requirements of the other elements present; and wherein the sum of b+c+e is greater than 1. European application No. 0,000,617 discloses a process for producing indene using a catalyst represented by the formula EQU M.sub.a P.sub.x O.sub.y
wherein M is one or more elements selected from Mg, Sr, Ca, Ba, La, Ce, other rare earths, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sn, Pb, Bi, Te, B, Al, Rh, Sb, As, U, Th, Ge and Ru; and wherein 0.1x.ltoreq..SIGMA.a.ltoreq.10x, wherein .SIGMA.a represents the sum of subscripts a of all of the metal ions and y is a number such that the valence requirements of the metal ions for oxygen is satisfied.